The two most common types of one way clutches found in automatic transmission applications are roller clutches and sprag clutches. Roller clutches have a plurality of caged rollers, each of which is located in a wedging pocket formed between a cylindrical pathway on one race and a sloped cam ramp on another race. Sprag clutches have a plurality of generally dumbbell shaped sprags located between two cylindrical pathways. Roller clutches are preferred to sprag clutches in many applications, both because of the lower cost of the clutch itself, and also because a roller clutch can tolerate a much greater degree of eccentricity between the clutch races, which allows for an easier clutch installation. This greater tolerance for race eccentricity results from the fact that the rollers are each individually spring energized, and can move independently up or down the cam ramps as the clutch overruns. Each roller, assisted by the continual bias by its respective energizing spring, automatically self seeks its own optimal position during clutch overrun. This optimal roller position can be termed the ready position, that is, the position where it is lightly engaged between the pathway and cam ramp, ready to quickly jam between the races. However, the ability of the roller to move up and down the cam ramp also creates some potential problems, especially in hign speed environments and environments where the rollers will be subjected to a high degree of external roller disturbing forces. These potential problems may be best illustrated by referring to FIGS. 1 through 3, which illustrate the structure and operation of a typical conventional roller clutch.
A conventional roller clutch, indicated generally at 10, includes a complement of cylindrical rollers 12, each of which is located in a wedging pocket formed between the cylindrical pathway 14 of an outer race 16 and a confronting cam ramp 18 formed on a substantially coaxial inner cam race 20. Clutch 10 has a cage 22 that forms its basic structural framework, and which is sized so as to be easily installed between the races 16 and 20, and tied to the cam race 20. Cage 22 is a fairly typical construction, and consists of a circumferentially spaced plurality of molded plastic journal blocks, one of which is indicated generally at 24, which are attached to a metal end ring 26. End ring 26 is partially broken away to show the roller 12 and journal block 24. Each journal block 24 is molded with a flat cross bar 28, which generally lies on a plane that is parallel with the axis of cage 22. A conventional energizing spring, designated generally at 30, has a generally square base mounting fold 32 clipped over cross bar 28, a roller conforming front contact loop 34 pressed against the side of a respective roller 12, and a serpentine central active portion 36 which consists of a series of V shaped folds. Cage 22 is the concentricity control type, meaning that the journal blocks 24 act as plain bearing members, and are sized so as to fit between the races 16 and 20 closely enough to keep the races substantially coaxial during overrun, but with enough clearance to allow for an easy installation of cage 22. This clearance, which is exaggerated for purposes of illustration in FIG. 1, also creates a running eccentricity between the races 16 and 20. During overrun, when the outer race 16 turns counter clockwise relative to inner race 20, the wedging pockets on one side of clutch 10 will widen, while those on the other side will narrow. The wedging pockets will not be of the same width at any instant of time, and over any time period, may each widen and narrow many times, especially at high speed. The race eccentricity is compensated for by the ability of each roller 12 to move rapidly up and down cam ramp 18 as its wedging pocket narrows and expands, generally referred to as roller travel. The energizing spring continually expands and contracts following the roller 12 as it so moves, and keeping it at ready position. A comparison of FIGS. 1 and 2 illustrates roller travel.
The forces that act on roller 12 during overrun, other than external roller disturbing forces, are those induced by the spring 30 and the manner in which it forces its roller 12 into the pathway 14 and the cam ramp 18. So long as roller 12 is kept in contact with with the pathway 14 and its respective cam ramp 18 by its spring 30, it will be maintained generally parallel to the race axis, its ideal orientation, regardless of its position on the cam ramp. The orientation of spring 30, however, will not be as ideally determined. The orientation of spring 30 is best considered in terms of its line of force, shown by dotted lines drawn between and perpendicular to both the center axis of roller 12 and the surface from which spring 30 pushes, that is, the surface of cross bar 28 that faces roller 12. The ideal or optimal line of force of spring 30 would be one which was directed more toward the cam ramp 18, rather than toward the pathway 14. This is because of the dynamic effects on the roller 12 at high speed overrun. The traction of the rapidly relatively rotating pathway 14 on roller 12, even if small in terms of percentage, can still result in a rapid spinning of roller 12, and consequent wear against cam ramp 18. Orienting the spring 30 so as to force the roller 12 more strongly toward cam ramp 18 than pathway 14 can help minimize roller spin. Furthermore, an optimal orientation for spring 30 would also be one in which all the pleats of the spring active portion 36 opened and closed equally and symmetrically about the spring center line, that is, the spring center line and spring line of force would be coincident, or at least close to it. This would minimize spring stress concentrations and oscillations and resist any tendency of the spring 30 to warp out of a straight line, maximizing spring life and stability.
With the conventional spring 30, however, that ideal spring orientation cannot be realized. The line of force of spring 30 will be determined by the position of roller 12 on the cam ramp 18, which changes dynamically, and by the location and orientation of cross bar 28 relative to roller 12. Cross bar 28, and thus the spring mounting fold 32. are fixed relative to cam ramp 18. As seen in FIG. 1, the orientation of cross bar 28 is such that, when roller 12 is located far up cam ramp 18, the spring line of force is directed toward pathway 14, and is offset a good deal from the spring center line, neither of which are optimal spring parameters. While the orientation of cross bar 28 could be initially tilted to direct spring 30 more toward the cam ramp 18 when the roller 12 was in the FIG. 1 position, to do so would threaten the spring operation when the journal block 24 was tight between the races 16 and 20, and the roller 12 was consequently located farther down cam ramp 18. Then, the roller 12 would have a tendency to move under the spring front loop, which could cause the entire spring 30 to pop up into the pathway 14 and lose contact with roller 12. This tendency of roller 12 to dive under spring 30 is worsened by the fact that external roller disturbing forces can actually cause roller 12 to move even farther down cam ramp 18, even out of contact with pathway 14, which is generally referred to as roller pop out, and shown in FIG. 3. It is not feasible to make spring 30 strong enough to resist roller pop out, as that would only worsen pathway traction. So, at no point is the line of force of conventional spring 30 optimal, either in terms of its direction or in terms of being close to the center line of the spring 30. While clutches like clutch 10 certainly work, their operation is not optimized.